﻿Vol. 66.] AROUND THE ROSS OF MULL GRANITE. 387 



is a very tough, quartzose, muscovite-biotite-gneiss, with a close- 

 texture and leaden colour. The sillimanite occurs throughout the 

 mass of the rock, and also in pale lenticular aggregates together 

 with some andalusite and cordierite. These lenticles are biconvex, 

 and commonly an inch or more in length and half an inch thick. 

 They lie along the foliation, often several being near together, and 

 there is a tendency for the rock to break along their convex 

 boundaries, which are faced with biotite. 



This rock is rendered conspicuous in the field by the weathering- 

 out of the sillimanite aggregates in the form of rugged knobs pro- 

 jecting an inch or more from the surface (see fig. 5, p. 386). These 

 knobs measure generally an inch or two across, and are about 

 2 inches apart. They consist of rough, fibrous, irregularly- 

 terminated prisms, often crossing one another. Most of these are 

 pink in colour, but some have a greenish tint. In size the prisms 

 may exceed 1 inch by half an inch, but commonly they measure 

 about a quarter by an eighth of an inch, and their roughness is pro- 

 bably due largely to the weathering away of other minerals from 

 among them. Some quartz and small garnets are also present. 



Sections of this rock, avoiding the lenticles, show it to consist of 

 quartz, twinned and untwinned felspars, brown biotite, Muscovites 

 and some magnetite and garnet, together with a variable amount 

 of apatite, cordierite, andalusite, fibrolite, and sillimanite prisms as 

 described below, as well as green spinel. 



The mutual relations of the common minerals are as usual in 

 schists, the relative proportions varying in different bands, while 

 the bands themselves are often coarsely rumpled. But the silli- 

 manite, which occurs at intervals, takes precedence over all the other 

 minerals, both for size and for perfection of crystal boundaries. 

 Fig. 6 (p. 388) shows a part of slice 14126 magnified 18 diameters : 

 the sillimanite is seen in cross-section in the central portion of the 

 photograph. 



Sections through the weathered-out knots, or the lenticles which 

 give rise to them, show sillimanite as the principal constituent. 

 There are also biotite, muscovite in large blades, patches of quartz, 

 garnet, magnetite, and a little felspar. In the lenticles andalusite 

 and cordierite are also plentiful, the former in one case [13971 J 

 being the chief constituent. Fig. 7 (p. 388) shows a portion of 

 a weathered-out knob [14127], and fig. 8 (p. 389) a portion of a 

 lenticle [14125]. In both of these sillimauite is the most con- 

 spicuous mineral. 



This segregation and crystallization of aluminium silicates in 

 such large proportion must have been accompanied by considerable 

 chemical reaction and recrystallization among the original con- 

 stituents of the rock. 



The sillimanite in these metamorphosed schists occurs in two 

 ways : — 



(1) Common fibrolite, present throughout the rock in felts, in bundles, and in 



groups of roughly-radiating fibres which penetrate the other minerals. 



(2) In -well-formed crystalline prisms. 



